Lenses with focal lengths bigger than 50mm are commonly known as telephoto models. These fit less in, and are therefore ideal for getting closer to distant subjects or picking out detail; they also give a more flattering effect when taking photos of people. In contrast to wide-angle, lenses with longer focal lengths have an inherently smaller depth-of-field, which means it’s easier to get a blurred background effect – again ideal for portrait, wildlife and sports photography.

Good focal lengths for portraits are typically between 85mm and 135mm – these are often known as short telephotos. Appropriate focal lengths for sports or wildlife are generally much longer – at least 200mm, and ideally 300mm or more. Professional sports and wildlife photographers often use 600mm lenses, or even longer still. These are known as super-telephotos.

Lenses with smaller f-numbers gather more light thanks to their bigger apertures, which makes them better in dim conditions and for minimising the depth-of-field. Bigger apertures however mean a larger, heavier and more expensive lens.

As focal lengths increase, the risk of camera-shake becomes greater, so telephoto zooms with image stabilisation are preferred. Canon refers to these models with the letters IS and these can let you handhold shots with shutter speeds three to four times slower than normal. Note these won’t stop a moving subject from blurring though, which brings many enthusiasts back to large aperture models.

As always, bear in mind any reduction in field due to your DSLRs sensor when choosing a lens. Canon’s APS-C models like the EOS 400D / XTi and 40D have a reduction factor of 1.6x, which effectively multiply lens focal lengths by 1.6 times. So if you’re after a portrait lens with an equivalent focal length of 85 to 135mm, you’ll need a model with an actual focal length of around 50 to 85mm.